1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, relates to an image forming apparatus capable of enhancing the resolution of an image by controlling a scan or process speed of the image forming apparatus having a laser scanning unit (LSU) that uses double laser diodes (LDs), and a laser scanning method thereof.
2. Related Art
Recently, as consumers' desire for faster printing speed has been increased, an image forming apparatus, such as a printer, a facsimile machine, a photocopier and a multi-functional product, has widely employed a multi-beam scanning approach of scanning two lines at a time using double laser diodes (LDs), rather than a single beam scanning approach of scanning a single line using a single LD.
If a multi-beam scanning approach is utilized, as shown in FIG. 1, an image forming apparatus emits laser beams with double LDs, i.e., upper and lower LDs, and scans a horizontal scanning signal (Hsync) once on a surface of a photosensitive body, such as a photoconductive drum, two dotted lines are simultaneously formed on the surface of the photosensitive body as an electrostatic latent image.
Further, when the multi-beam scanning approach is utilized, the upper and the lower LDs are arranged to be vertically fixed to each other at a certain interval. These intervals between dots of a scan line (dotted line shown in FIG. 1) can be adjusted to be narrow or broad by controlling a correspondent signal of each of the LDs. However, the intervals between the scan lines cannot be adjusted by controlling a corresponding signal of each of the LDs.
Accordingly, the intervals between scan lines of laser beams emitted from the upper and the lower LDs, i.e., scan lines of a scanning signal (Sync) need to be narrow so as to improve a vertical resolution of an image.
If a single-beam scanning approach is utilized using a single LD, intervals between scan lines of a scanning signal (Sync) can be adjusted by controlling a scan speed of a laser beam emitted from a single LD on a surface of a photosensitive body or a process speed of an engine driving unit (EDU) of the image forming apparatus. On the other hands, if a multi-beam scanning approach is utilized using double LDs, there is produced superposition or interference between scanning signals by controlling a scan speed or a process speed, so that it is difficult to be implemented.
More specifically, FIG. 2 illustrates scan lines formed on a surface of a photosensitive body in the case that a scan speed of a laser scanning unit (LSU) is increased double or a process speed of an engine driving unit (EDU) is decreased ½ time to enhance a vertical resolution of an image when compared with those shown in FIG. 1. In this case, as shown in FIG. 2, scan lines of scanning signals (Sync) having image information different from each other, which are emitted from the upper and the lower LDs, are mixed up. As a result, a required high-resolution image cannot be obtained.
Accordingly, there is a need for a laser scanning method and an image forming apparatus capable of obtaining a required high-resolution image without superposition or interference between scan lines of scanning signals (Sync) even though a scan speed of a laser scanning unit (LSU) or a process speed of an engine driving unit (EDU) are changed to enhance a vertical resolution by narrowing intervals between scan lines of scanning signals (Sync).